This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned micro-spring structures formed from stress-engineered metal films.
Photo lithographically patterned spring structures (sometimes referred to as xe2x80x9cmicrospringsxe2x80x9d) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical microspring includes a spring metal finger having an anchor portion secured to a substrate, and a free (cantilevered) portion extending from the anchored portion over the substrate. The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The free portion of the spring metal finger bends away from the substrate when the release material located under the free portion is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The present inventors have observed that conventional spring structures develop contact resistances that are detrimental to signal transmissions when the spring structures are used as conductors. The spring metal (e.g., Mo, MoCr, NiZr) is typically chosen for its ability to retain large amounts of internal stress. These materials typically oxidize in air, a phenomenon that can interfere with their ability to make electrical contact, for example, with the contact pad of an integrated circuit when used in a probe card. The spring metal materials can also gall to the contact pad, which is typically aluminum. Once the galled aluminum oxidizes, the contact resistance between the contact pad and the spring metal structure increases. One proposed approach to reducing contact resistance is to passivate the spring metal before etching and release. However, the passivating material tends to resist bending of the spring metal finger after release, and provides minimal coverage along the front edge at the tip, thereby allowing direct contact with the spring metal that can result in increased contact resistance.
What is needed is a spring metal structure that resists increased contact resistance by avoiding oxidation of the spring metal and/or galling of a contact pad against which the spring metal structure is pressed.
The present invention is directed to efficient methods for fabricating microspring structures in which a conductive coating is deposited on the tip of the free (i.e., cantilevered) portion of the spring metal finger using a directional deposition process after release from an underlying substrate. By directing the spring coating deposition on the spring metal finger tip after release (i.e. after the finger is allowed to bend upward from the substrate due to internal stress), the conductive coating is reliably formed on the front edge and upper surface of the spring metal finger tip without impeding the bending process, thereby producing a low-cost spring structure with reduced contact resistance when compared to non-coated spring structures, or to spring structures coated before release.
In accordance with the disclosed method, a conductive release layer is deposited on a substrate, and then a stress-engineered (spring) metal film is formed on the release material layer. A first mask is then used to etch an elongated spring metal island from the metal film, but etching is stopped before the release layer is entirely removed to prevent undercutting that can cause premature release of the spring metal island. A release (second) mask is then deposited that defines a release window exposing a portion of the spring metal island and the release material layer surrounding this exposed portion. In accordance with an aspect of the invention, the release window is formed with an overhang that helps prevent overlapping of coating material, thereby facilitating lift-off of the residual coating formed on the release mask. Subsequent removal of the release material exposed by the release mask causes the exposed portion of the spring metal island to bend away from the substrate due to its internal stress, thereby becoming the free portion of a spring metal finger (an anchored portion of the spring metal finger remains covered by the release mask). The release mask is then used as a mask during the deposition of the conductive coating (e.g., a refractory noble metal such as Rhodium (Rh), Iridium (Ir), Rhenium (Re), Platinum (Pt), and Palladium (Pd)) on the tip and other exposed portions of the spring metal finger. The overhanging release mask structure prevents overlapping of the coating material to facilitate lift-off of residual coating portions during the subsequent removal of the release mask.
In another embodiment, the release mask, which is also used during the deposition process, is provided with a channel extending over the anchored (i.e., non-released) portion of the spring metal finger, thereby facilitating the formation of conductive coating portions on the anchor portion of the spring metal finger to improve conductivity.